You MOT your car annually and maybe get it serviced once or twice a year. Other than on those occasions, how often do you check its safety?
Next time you go to your car, take a minute to check the tread on the tyres, and their pressures. Check all your lights (use a shop window, for example, to check your brake lights) and try the horn. Obviously, these quick tests are no substitute for regular preventative maintenance.
It is worth thinking more about the purpose of your tyres. With just four, or even two, rather small contact patches they allow us to stop relatively quickly and navigate bends in the road safely. Maintaining the grip they require when water gets between rubber and road is difficult, so the grooves of the tread provide a means for that water to be squeezed out of the way. The deeper the grooves are, the more efficiently the water leaves the tyre.
The legal minimum tread depth for car tyres is 1.6mm (shown by small blocks of rubber set into the tyre's grooves). However, it's worth noting that police cars travelling to emergencies have their tyres replaced much closer to 3mm.